1. Field of the Invention
This invention relates to a electronic ballast, and more particularly to a electronic ballast with crest factor correction.
2. Description of the Prior Art
A crest factor (C.F.) of lamp current in a fluorescent lamp is defined as the ratio of a peak current of current wave packet in a lamp and a RMS (root mean square) current thereof after the lamp has been ignited. But, many high frequency signals hides in the AC (alternating current) current and so the C.F. value is larger than the ideal value of 1.414. The value of the C.F directly effects a life span of the lamp. Many major lamp manufactures have imposed a maximum value of 1.7, and high C.F. values will shorten the normal life span of the fluorescent lamp.
Electronic ballasts with higher power factor correction (PFC) are being imposed worldwide in order to maximum the efficiency of existing power generation capacity. Adding additional power plant is expensive and increasingly difficult and the burden of good quality power actually falls on the power user rather than the power supplier. Therefore, a cost effective method of power factor correction is a major task for many electronic ballast engineers.
There are two basic types of PFC circuits, namely active PFC circuit and passive PFC circuit. FIG. 1 shows a typical active PFC circuit, wherein the symbol of 10 represents a bridge rectifier. It uses a boost topology and a high frequency switching MOSFET to achieve high power factor. The boost topology of FIG. 1 may be operated at variable frequency mode with continuous inductor current or in the critical conduction mode where the inductor is allowed to discharge to zero energy in before initiating a new charge cycle. The active PFC circuit is entirely satisfactory for harmonic compliance and so is capable of achieving a power factor up to 99%. However, the material cost is high since it requires a controller PFC, an inductor L1, a high-speed power MOSFET Q1 and a fast recovering diode D1.
On the other hand, the passive PFC circuit uses only passive parts and cost less than the active PFC circuit. FIG. 2 shows a simple L-C passive PFC circuit. This type of circuit operates at main frequency (50 Hz or 60 Hz) using PFC choke L1 and capacitor C1 tuned to the main frequency in a low pass filter configuration. Unfortunately, due to operating at mains frequency, a large capacitance and a large inductance are required, especially the physical size and weights of the inductor L1 is very big and a unpleasant buzz is occurred. Hence, the passive PFC is seldom used now although it is capable of achieving a good performance.
Recently, there are many low cost PFC circuits, for example: valley fill PFC circuit. FIG. 3A shows a valley fill PFC circuit achieving a high power factor of 93%-95%. In this circuit, the filter capacitors C1 and C2 are charged in series via the diode D2 on each half cycle of the rectified AC input. Each capacitor is charged to xc2xd of the AC peak voltage (assuming no voltage drop across diodes D1, D2, and D3). Since each capacitor is charged up to xc2xd of the AC peak voltage, the capacitors C1 and C2 supply a output current only after the DC rail follows the sinusoidal waveform down to xc2xd of the AC peak voltage. At this time, the capacitors C1 and C2 are essential in parallel and supply the load current until the rectified AC input exceeds xc2xd of the AC peak voltage again at next half cycle (assuming the capacitances of the capacitors C1 and C2 being very large and so the voltage of the capacitors C1 and C2 not decreases after discharged). The waveform of the output DC voltage is shown as FIG. 3B. By a simple calculation, the ratio of the time when the value of a sinusoidal waveform is less than xc2xd of the maximum value thereof and total time is about 37%, namely the discharge duty cycle of capacitors C1 and C2 is around 37% followed by an idle period of around 63%. During the idle period the load is being supplied directly from the rectified AC input. This greatly increases the power quality and increases the power factor. The drawback of valley fill PFC circuit is the big ripple (xc2xd of AC input) voltage at the DC output. Any electronic ballast using such DC bus without regulating the lamp current will result in a higher valve of crest factor. Most low cost half-bridge MOSFET drivers or controllers used in electronic ballast for a fluorescent lamp don""t have the capability of regulating the lamp current. In most cases, the operating frequency of the driver is fixed after the timing capacitor and resistor has been chosen. Any electronic ballast that is designed using such low cost controller or MOSFET driver, the lamp current is not regulated. Hence, if a voltage of a DC bus varies supplying to the electronic ballast, the lamp current also varies with the DC ripple. It is possible that the crest factor of lamp current might be higher than 1.7 and shorten the lamp life span. Hence, the valley fill PFG circuit is not suitable for the low cost electronic ballast without the capability of regulating the lamp current. FIG. 4A shows a typical electronic ballast using valley fill PFC without regulating lamp current, wherein the symbol of 20 represents a half-bridge MOSFET driver, the symbol of 30 represents a lamp. The current waveform of the lamp 30 is shown as FIG. 4B. The C.F. value of lamp current can easily go much higher than 1.7.
In those conventional arts, the low cost electronic ballast with high power factor does not have the capability of regulating the lamp current and so the life span of the lamp is shortened. Moreover, the cost of the electronic ballast having the capable of high power factor and regulating the lamp current simultaneously is very high. One of objectives of the present invention is to provide simple low cost electronic ballast controller for electronic ballast with capable of crest factor correction for efficiently lengthening the life span of a lamp.
Another objective of present invention is employ a electronic ballast controller to correct the crest factor of lamp current for reducing the crest factor of the lamp current.
As aforementioned, the present invention provides an electronic ballast. The electronic ballast comprises a full-wave rectifier, an inverter, a power factor correction means, a voltage signal generator, a controller, a L-C resonant circuit. Wherein when a rectified voltage increases, the inverter increases a resonant frequency of a output AC voltage according to a switch signal for reducing a current passing through a inductor of the L-C resonant circuit, and when the rectified voltage decreases, the inverter decreases the resonant frequency of the output AC voltage according to the switch signal for increasing the current passing through the inductor.
The present invention also provides an electronic ballast with crest factor correction for a fluorescent lamp. The electronic ballast comprises a full-wave rectifier, an inverter, a valley fill circuit, a voltage signal generator, a controller, and a L-C resonant circuit. Wherein when a rectified voltage increases, the inverter increases a resonant frequency of a output AC voltage according to a switch signal for reducing a current passing through a inductor of the L-C resonant circuit, and when the rectified voltage decreases, the inverter decreases the resonant frequency of the output AC voltage according to the switch signal for increasing the current passing through the inductor.
Compared with the low cost electronic ballast with high power factor does not have the capability of regulating the lamp current in those conventional arts and so the life span of the lamp is shortened. On the other hand, the cost of the electronic ballast having the capable of high power factor and regulating the lamp current simultaneously is very high. The present invention employs a voltage feedback signal to adjust the operating frequency for simply controlling the lamp current. Hence, a simple low cost electronic ballast controller with capable of crest factor correction is used for electronic ballast to efficiently reduce the crest factor of the lamp current and lengthen the life span of a lamp.